Field of the Invention
The invention relates to a thick film sensor for hydrogen and carbon monoxide and a method for preparing such a device. More particularly, the invention is a gas detecting element having a high degree of combustible gas sensitivity. The element is an n-type semiconductor oxide, bismuth molybdate, that does not require the presence of a catalyst.
There is a continuous need to detect pollutant gases and control the combustion efficiency in fuel burning chambers. This need has prompted the investigation of many combustible gas detection methods as well as the investigation of materials which can function as gas detecting elements in such methods. Recent advances in the field of heterogeneous catalyses have increased the interest in electronic sensing elements. Both n-type and p-type semiconductor oxide films are examples of electronic sensing elements which undergo measurable changes in their electronic conductivity, .sigma., on exposure to reducible gases in the presence of oxygen-containing atmospheres. The changes in .sigma. can be related to the concentration of combustible gases such as hydrogen and carbon monoxide and can be measured to determine concentration.
Heretofore, it has been the conventional belief that the conductivity changes in semiconductor oxide films became large enough to measure only if at least small amounts of a noble metal bearing compound catalyst are added to the semiconductor oxide film material. Stannic oxide has been found to be a particularly useful semiconductor oxide when it is mixed with small amounts of a noble metal catalyst such as platinum, palladium and rhodium. An example of an improved thick film stannic oxide sensor which is enhanced through the use of a selective catalyst is disclosed in U.S. Pat. No. 4,397,888, which is assigned to the present assignee and the contents of which are incorporated herein by reference.
It is therefore an object of this invention to provide an electronically active n-type semiconductor oxide material for the fabrication of a thick film gas sensing element.
It is another object of this invention to provide a gas sensor device for use in both pollution and gas boiler control applications.
It is a further object of this invention to provide an n-type semiconductor oxide material which does not require the presence of any catalyst for useful combustible gas response.
It is still another object of this invention to provide a sensor device characterized by fast, reproducible and reversible response to hydrogen and carbon monoxide as well as being characterized by ease and economy in manufacture.